The present invention relates to a novel polymeric conjugate of a methotrexate derivative and a pyran copolymer exhibiting an excellent anticancer effect and a method for the preparation thereof.
Methotrexate is a known anticancer agent with wide therapeutic applications exhibiting remarkable effectiveness against acute lymphocytic leukemia, mammary cancer, lung cancer, head and neck cancer and the like. It is an antagonist of folic acid and has an activity to kill cells by the inhibition of DNA synthesis when it is strongly bound with the dihydrofolate reductase.
Along with the excellent anticancer activity, however, methotrexate exhibits serious side effects such as gastrointestinal tract disorder, decrease of leukocytes and blood platelets, renal disfunction and the like due to the toxicity against normal cells at a growing stage. Therefore, it is necessary that methotrexate must be clinically administered avoiding the possible side effects.
When a cancer patient is administered with a low-molecular anticancer agent such as methotrexate, in general, the concentration thereof in blood is rapidly decreased with a relatively short half life time. While the anticancer activity of methotrexate is a consequence of the activity thereof to the S phase in the cell cycle of cancer cells, 36 to 48 hours, the anticancer effect obtained by methotrexate is limited. Although the concentration of the agent in blood can be sustainedly increased to be effective on cancer cells by increasing the dose, this way cannot be undertaken for methotrexate because the concentration thereof in blood may be unduly high within a period so as to cause damages on normal cells such as mucosa cells at the growing phase.
It is a natural idea that the anticancer effectiveness of an anticancer agent such as methotrexate can be enhanced without increasing the toxicity when the anticancer agent is sustainedly released into the blood by some means so that the concentration thereof in blood could be maintained in such a level as to exhibit the desired anticancer effect with minimum side effects. Known methods for sustainedly releasing a drug into the patient's blood include the direct method such as the continuous infusion clinically undertaken heretofore and the indirect method in which the anticancer agent is administrated as incorporated into liposomes or polymeric microspheric particles to avoid the damage on the patient's body. The latter method is highlighted as an excellent method for the administration of a drug with strong side effects because the concentration thereof in blood can be effectively controlled by a single administration. The so-called "drug delivery system" (DDS) including these methods or concepts is under rapid progress in recent years in the field of medical and pharmaceutical science. The basic principle of DDS is to give a necessary drug to the appropriate position of the body, at an appropriate time and in an appropriate concentration. Utilization of a conjugate of a drug and a polymeric compound as a carrier is also a promising method of DDS in respect of the long half life time of the polymeric compound in the body and the sustained releasability of the drug from the polymer conjugate. This means of sustained release of a drug by forming a polymer conjugate would be particularly effective for a drug such as methotrexate of which enhancement of the anticancer effect and decrease of the side effects can be expected by controlling the concentration thereof in blood at an appropriate level.
Various proposals and attempts have been made heretofore for methotrexate from this standpoint of conjugate formation. For example, a conjugate thereof with serum albumin was reported recently in Neoplasma, volume 37, pages 343-349 (1990). Since albumin is a protein derived from a living body consisting of a large number of amino acid residues, there is no information on the type and sites of the chemical bond between albumin and methotrexate. As a consequence, the anticancer activity of such an albumin conjugate of methotrexate is almost identical against B16 melanoma with that of methotrexate as such. In short, no polymeric conjugate known heretofore can exhibit a superior anticancer activity to that of methotrexate per se.
Despite the so great possibility of enhancement of the anticancer effect to be obtained by the sustained drug relesability with a conjugate of methotrexate and a polymer as is expected from the pharmacological mechanism, no effective means is reported heretofore by which a conjugate of methotrexate with a polymer can exhibit higher anticancer activity than that of methotrexate as such. One of the reasons therefor is the absence of precise information or knowledge which enables definitively designing the bonding type between the molecules of the polymer and methotrexate to meet the particular requirements. It would be an idea therefore that the glutamic acid residue at the end of the methotrexate is utilized for bonding with a polymer molecule so that the desired sustained releasability of the drug could be established by selecting the type of the polymer to obtain higher anticancer activity than methotrexate as such.